Load cells are used to provide accurate measurements of compressive or tensile forces. Typically, the force creates a strain in the load cell which is measured by strain gage transducers. Accurate measurements, however, require that the force be applied along an axis which is central to the load cell and about which all the transducers are symmetrically placed.
An example of such a load cell which is sensitive to the effects of off-axis loading is known as a compression washer. U.S. Pat. No. 4,212,360 discloses such a load cell (FIGS. 4a and 4b), an example of which is shown in a product brochure entitled "Compression Only/Thru Hole Load Washer". As the name implies, this type of load cell is configured as a washer so that it can be mounted by securing a bolt through the device.
Other compressive load cells which are less sensitive to off-axis loading are characterized by several types. One such load cell employs a shear web design disclosed in U.S. Pat. No. 5,461,933. This design consists of a pair of concentric rings joined by two or more web members. Transducer elements disposed on the webs provide compression and tension force measurements. Although this design reduces sensitivity to off-axis loads, the web assembly must be accurately machined to tight tolerances. In addition, the traducers must be carefully bonded to the walls of the webs in a symmetrical manner in order to ensure accuracy in the measurements.
A ball and socket assembly design serves to maintain the loading force along the principle axis and exactly centered on the load cell. Such a design is shown in Photo 2 in the article by Clegg entitled "Bonded Foil Strain Gauge Force Transducers", Sensors, October 1996, pp. 68-75. As can be seen from the photograph, the manufacture of such a device is can be quite expensive.
A shear beam load cell, such as the one disclosed in U.S. Pat. No. 5,220,971, can be also be used in tension and compression applications. The transducer elements are diagonally placed on a machined web in the center portion of the device and measure the shear force of the applied load. Such devices are expensive to machine and accurate measurements depend on consistently centered and axially constrained forces.
In patent application Ser. No. 08/795,593 owned by the owner of the present invention and incorporated herein by reference, a helical coil load cell is disclosed which addresses the shortcomings of the described prior art load cells. The helical load cell, however, exhibits sensitivity to side-loading and torsional loads. For example, a load that is applied along a line that is not parallel to the central axis of the helical coil, presents a horizontal force component known as a side load. In addition to the side loading force, a torsion load is produced in the presence of forces which tend to "unwind" or "close down" the coil. It is desirable, therefore, to provide an improved helical load cell which is insensitive to such side loading and torsional loading forces.